As illustrated in FIG. 6, solar cell modules usually have a structure in which a solar cell 1 is sealed in a sealant layer 2, and the sealant layer 2 is sandwiched by a surface layer 3 made of a material such as glass or a transparent resin and a weather-resistant sheet 4. Here, the sealant used is an ethylene/vinyl acetate copolymer (EVA).
The weather-resistant sheet 4 in a solar cell module is provided for an increase in the mechanical strength of the module and for prevention of moisture (vapor) entry into the sealant layer 2.
The weather-resistant sheet 4 includes, as illustrated in FIG. 7, a water impermeable sheet 5 for providing a vapor barrier property, and a resin sheet 8 that is attached to one surface of the water impermeable sheet 5. Usually, a resin sheet 9 is attached to the other surface of the water impermeable sheet 5.
The water impermeable sheet 5 is produced from a material such as a Si-deposited polyester (Si-deposited PET) and a metal (e.g., aluminum, stainless steel) which are excellent in water impermeability, and the sheet usually has a thickness of 10 to 20 μm.
The resin sheets 8 and 9 are desired to have properties such as weather resistance, electrical insulation, fire retardancy, and a design property, and are sheets produced from polyvinyl fluoride polymer (PVF). A polyethylene sheet may be used as the resin sheet 8 which is on the sealant layer 2 side.
These resin sheets, however, are usually required to have a thickness of 20 to 100 μm in order to satisfy the required properties such as weather resistance and electrical insulation, which leads to an increase in the weight. Hence, the sheets are desired to be further reduced in weight.
In this context, Patent Document 1 and Patent Document 2, for example, have proposed formation of a similar layer with use of a resin coating material instead of a resin sheet.
In Patent Document 1, an epoxy resin coating material is used as a resin coating material. However, epoxy resin does not have sufficient weather resistance, and thus has not been practically used.
In Patent Document 2, a weather-resistant sheet is proposed which has a double-layer structure formed by applying a PVdF coating material, produced by mixing PVdF free from functional groups with a specific amount of tetraalkoxysilane or a partial hydrolysate thereof, to a metal substrate (water impermeable sheet). Since PVdF in the PVdF coating material does not have functional groups, the PVdF coating material has low adhesion to EVA (sealant) when used alone. The adhesion is increased in Patent Document 2 by mixing a specific amount of tetraalkoxysilane or a partial hydrolysate thereof in the coating material, and applying tetraalkoxysilane or a partial hydrolysate thereof to the interface between the coating material and EVA. However, the structure does not contribute to an increase in the adhesion between the PVdF coating film and the metal substrate because the tetraalkoxysilane or a partial hydrolysate thereof is unevenly distributed in the interface with EVA. Also, since PVdF is crystalline and does not have functional groups, the PVdF coating material is required to be heated at 200° C. to 300° C. for 20 to 120 seconds to be formed into a coating film. For this reason, it has been difficult to use PVdF on a water impermeable sheet other than metal ones.
Patent Document 3 has proposed a weather-resistant sheet for a solar cell which solves the above problem, i.e., a weather-resistant sheet having excellent adhesion to a water impermeable sheet regardless of the type of the water impermeable sheet, in which a cured coating film layer of a hydroxyl group-containing fluoropolymer coating composition is formed on at least one surface of a water impermeable sheet.
Here, the actual production steps include a step of rolling a water impermeable sheet having a coating film layer formed thereon (a rolling step) for an increase in the production efficiency. If the partially cured coating film has tackiness in the rolling step, the back surface of the rolled water impermeable sheet and the surface of the coating film layer may cause a problem of blocking as illustrated in FIG. 8. The reference number 5 indicates a water impermeable sheet, and the reference number 7 indicates a coating film.
In the case of the hydroxyl group-containing fluoropolymer coating material of Patent Document 3, a weather-resistant sheet consisting of a water impermeable sheet and a coating film layer is dried at 80° C. for 30 minutes, and the adhesion between the coating film and the sheet is evaluated. Here, the adhesion and the blocking resistance between the back surface of the water impermeable sheet and the surface of the coating film layer in the rolled state can be further improved. Also, adhesion evaluation is made on a sample produced by disposing an EVA resin sheet on the surface of the coating film of the weather-resistant sheet and pressing the sheets, and the adhesion here can also be further improved.
Patent Document 1: JP 7-176775 A
Patent Document 2: JP 2004-214342 A
Patent Document 3: JP 2007-035694 A